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Date: Tue, 11 Sep 2018 16:43:04 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20180911204304.GA20131@flamenco>
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	<f6d7f508-eee6-cc53-248c-9553b68d15f5@twiddle.net>
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Subject: Re: [Qemu-devel] [PATCH v2 02/12] util: add atomic64
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To: Richard Henderson <rth@twiddle.net>
Cc: qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Eduardo Habkost <ehabkost@redhat.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Murilo Opsfelder Araujo <muriloo@linux.ibm.com>

On Tue, Sep 11, 2018 at 05:43:38 -0700, Richard Henderson wrote:
> On 09/10/2018 04:27 PM, Emilio G. Cota wrote:
> > +#define GEN_READ(name, type)                    \
> > +    type name(const type *ptr)                  \
> > +    {                                           \
> > +        QemuSpin *lock = addr_to_lock(ptr);     \
> > +        type ret;                               \
> > +                                                \
> > +        qemu_spin_lock(lock);                   \
> > +        ret = *ptr;                             \
> > +        qemu_spin_unlock(lock);                 \
> > +        return ret;                             \
> > +    }
> > +
> > +GEN_READ(atomic_read_i64, int64_t)
> > +GEN_READ(atomic_read_u64, uint64_t)
> 
> Is there really a good reason to have two external
> functions instead of having one of them inline and
> perform a cast?

Not really. I can do a follow-up patch if you want me to.

> Is this any better than using libatomic?

I didn't think of using libatomic. I just checked the source
code and it's quite similar:
- It uses 64 locks instead of 16 ($page_size/$cache_line,
  but these are hard-coded for POSIX as 4096/64, respectively)
- We compute the cacheline size and corresponding padding
  at run-time, which is a little better than the above.
- The locks are implemented as pthread_mutex instead of
  spinlocks. I think spinlocks make more sense here because
  we do not expect huge contention (systems without
  !CONFIG_ATOMIC64 have few cores); for libatomic it makes
  sense to use mutexes because it might be used in many-core
  machines.

So yes, we could have used libatomic. If you feel strongly
about it, I can give it a shot.

Thanks,

		Emilio